This invention relates to a process for removing boron and fluoride ions from water generated in mine dewatering operations so the ultimate boron and fluoride levels meet stringent governmental environmental requirements for discharge of such water into surface streams.
As a particular example, water resulting from dewatering operations at gold mines in the Carlin trend area of Nevada, U.S.A. can contain dissolved boron concentrations on the order of about 1 mg/L, which due to environmental regulations must be reduced to about 0.6 mg/L or less before such water can be dispensed into the Humboldt River. Such water may also contain dissolved fluoride ion concentrations on the order of about 1.5 mg/L, which must be reduced to about 0.8 mg/L or less.
Prior attempts to remove boron from water by conventional water treatment methods such as treatment with aluminum sulfate, ferric salts, and lime have proved to be ineffective. Evaporation-crystallization processes and solvent extraction processes have been investigated. Ion exchange processes employing strong base ion exchange resins have been demonstrated to be effective, but remove other ionic species as well. As such, they are inefficient if boron is the only element to be removed. One resin, the ion specific resin for boron developed by Rohm & Haas Co., is also uneconomic for treatment of large volumes of water containing a low B concentration.
Mine dewatering operations are found in conjunction with, for example, gold recovery operations in the Carlin trend area of Nevada, U.S.A. Processes for the recovery of gold from refractory sulfidic ores, "double refractory" ores containing sulfidic and carbonaceous material, and other difficult ores such as those ores located in the Carlin trend employ pressure oxidation under acidic conditions as disclosed, for example, in Thomas et al. U.S. Pat. No. 5,071,477 and Thomas et al. U.S. Pat. No. 5,489,326 and/or, alternatively, a roasting operation. The products of such oxidation processes typically contain high acidic contents which must be neutralized before further processing by cyanidation as disclosed in the Thomas et al. patents, or by thiosulfate leaching in the manner disclosed in Marchbank et al. U.S. Pat. No. 5,536,297. There is a need, therefore, for an abundant source of neutralizing agent which is compatible with the gold recovery process in that it does not contain contaminants or other agents which substantially interfere with gold recovery, and which does not add substantially to the raw material requirements and disposal requirements for the overall operation.